The present invention relates to an alignment apparatus mounted on an exposure apparatus, an aberration measuring method and an aberration measuring mark.
Alignment apparatuses mounted on exposure apparatuses used in manufacturing semiconductor devices are classified into three systems. In the first system, a laser beam is radiated on an alignment mark, and light scattered from the alignment mark is detected. In the second system, an image of an alignment mark obtained through an enlarging optical system is projected on a light receiving surface of a CCD camera, and the position of the alignment mark is measured by the image processing technique. In the third system, heterodyne interference is utilized. For example, the LSA sensor of Nikon Corp. utilizes the first system, the FIA sensor of Nikon and the alignment sensor of Canon Inc. utilize the second system, and the LIA sensor of Nikon Corp. and ATHENA of ASML utilize the third system.
The performance of an alignment apparatus required in the actual manufacture of semiconductor devices is judged in two phases. In the first phase, a capability of detecting a signal without fail and a high general-purpose property are required. In the second phase, measurement accuracy and accuracy stability are required. The alignment apparatus of the first system is advantageous in the first phase; i.e., it has a high signal-detecting capability and a high general-purpose property. However, as the size of a semiconductor device is reduced, the alignment apparatus of the first system cannot have fully satisfied requirements for the accuracy in the second phase.
For this reason, expectations for the alignment apparatus of the second system have increased. In other words, so far as an alignment signal is detected, a higher alignment accuracy can be expected in the second system than in the first system. Therefore, the alignment sensor of the second system will be used positively in future, as the low general-purpose property is compensated for by modification of the design of the alignment mark or improvement of the cross-sectional structure.
FIG. 10 is a schematic diagram showing an alignment apparatus of the second system to which the present invention is applied. A light beam emitted from a halogen lamp 2 is radiated on an alignment mark on a substrate 4 through an illuminating optical system 1 and an objective 3. An image of the alignment mark is formed on a light receiving surface of a CCD camera 6 by means of an enlarging projection optical system 5. FIG. 1 shows a mark generally used as the alignment mark. The mark has seven stripe patterns each having a 6 .mu.m width, which are arranged side by side with a pitch of 12 .mu.m. Each stripe pattern has a grooved or projecting cross-sectional structure. The position of the alignment mark is detected by an edge signal of the 6 .mu.m pattern.
To overcome the drawback of the alignment apparatus of the second system, i.e., the low general-purpose property, a document 1 (SPIE vol. 3051, pp. 836-845) reports an improved alignment apparatus wherein the detection capability is increased by an alignment mark with a small difference in level. At present, the phase-shift alignment apparatus disclosed in the document 1 is practically proposed.
The alignment apparatus disclosed in the document 1 can detect a satisfactory signal from an alignment mark of low contrast with a small difference in level. Specifically, a light stop 8 and a 180.degree. phase plate 9 are arranged at positions as shown in FIG. 10. Only the light passed through the central portions of the lenses is 180.degree. phase-shifted. As a result, alignment can be achieved by an alignment mark, from which a satisfactory alignment signal could not be obtained by a conventional apparatus.
The light stop 8 and the phase plate 9 are shown in FIGS. 2A and 2B. FIG. 2A is a plan view of the light stop 8 and the FIG. 2B is a plan view of the phase plate 9. As shown in FIGS. 2A and 2B, the light stop 8 and the phase plate 9 have similar figures. A light shielding portion 91 of the light stop 8 corresponds to a 180.degree. phase shifter of the phase plate 9.
When alignment is performed by the second system, the measurement value of the position of the mark may vary due to so-called manufacture errors, such as the aberration of the enlarging optical system or the deviation of the optical axis. The variation of the measurement value, i.e., the amount of an error in measured position, is considered to be a factor of the alignment offset. Further, it is considered that the alignment offset varies depending on not only the alignment sensor but also the design or the cross-sectional structure of the mark. In other words, the offset includes an alignment offset generated due to a difference in structure between alignment marks in the same exposure apparatus, as well as an alignment offset which varies from one exposure apparatus to another.
In alignment using the same alignment mark, it is considered that the variation of alignment offset among alignment apparatuses results from the aberration in the optical system for projecting an image of the alignment mark on the detector. Similarly, in alignment using the same alignment apparatus, it is considered that the variation of alignment offset among alignment marks of different structures results from the aberration of the optical system.
Conventionally, the alignment offset as described above is avoided by previously providing different correction values in advance for different alignment sensors or different mark structures. Recently, however, the alignment accuracy has been required to be higher and higher. In addition, there has been increasing cases wherein different exposure apparatuses are mixed. Under the circumstances, it is difficult to obtain the required alignment accuracy. Therefore, it is increasingly important to eliminate the alignment offset itself.
A document 2 (Jpn. J. Appl. Phys. Part 1, No. 12B, Vol. 36 (1997) pp. 7512-7516) reports that an alignment offset results from an interaction between aberration or an adjustment error of an alignment apparatus and a cross-sectional shape or a layer structure of an alignment mark. In the document 2, a difference in level of the alignment mark is noticed as a cause of the alignment offset, and a height of the alignment mark and symmetry of an alignment signal are searched.
According to the conventional art, a lens is independently adjusted in advance, and thereafter incorporated into an alignment apparatus. After incorporated, since the lens is adjusted on the basis of symmetry of the alignment signal at the test mark, it is difficult to obtain an accuracy as high as that of the independent adjustment.
As described above, when the conventional alignment apparatus performs alignment, the measurement value of the position of the mark varies due to the aberration of the enlarging optical system. The measurement value varies not only among exposure apparatuses. Even in the same exposure apparatus, the measurement value may vary due to a difference in structure of the alignment mark.